1. Field of the Invention
The present invention generally relates to high frequency, loop antennas and, particularly, to such antennas having a series reactance in the loop.
In the past, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of the lowest operating frequency. These dimensions allowed the antenna to be excited easily and to be operated at or near resonance, limiting the energy dissipated in impedance losses and maximizing the transmitted energy. These antennas tended to be large in size at the resonant wavelength, and especially so at lower frequencies.
2. Discussion of the Related Art
In order to address the shortcomings of traditional antenna design and functionality, the meander line loaded antenna (MLA) was developed. One such antenna is disclosed in U.S. Pat. No. 5,790,080 for MEANDER LINE LOADED ANTENNA, issued to John T. Apostolos, the inventor of the present application, the contents of which are hereby incorporated by reference.
The aforementioned U.S. Pat. No. 5,790,080 describes an antenna that includes two or more conductive elements acting as radiating antenna elements, and a slow wave meander line adapted to couple electrical signals between the conductive elements. The meander line has a variable physical length which affects the electrical length and operating characteristics of the antenna. The electrical length of the meander line, and therefore the antenna, may be readily controlled.
More specifically, such an antenna includes two, spaced-apart vertical conductors and a horizontal conductor. The vertical and horizontal conductors are separated by gaps, which are bridged by meander lines. The meander lines include a slow wave structure having sequential sections with alternating high and low impedance values, which structure provides an electrical length that is greater than its physical length.
A meander line 108 according to the prior art is shown in FIG. 1 and is characterized by a plurality of series connected sections 110, 112. Sections 110, 112 are alternately sequentially connected and are designed to have respective high and low characteristic impedance values, which impedance values are sequentially alternated by the alternating sequential connection. These alternating impedance values create a slow wave structure having an effective electrical length that is greater than the actual physical length. This impedance structure may be formed by a transmission line having sections which alternate in their separation from a ground plane. In FIG. 1, high impedance sections 110 are suspended above the top surface of a dielectric sheet 114 and low impedance sections are formed as conductors directly on the top surface of dielectric sheet 114. Placing the dielectric sheet against a large planar conductor creates the different impedance values because the planar conductor acts as an effective ground plane. In the prior art antenna, the vertical or horizontal conductors of the antenna may be used to create that ground plane for meander line 108.
Meander line 108 is also designed to allow adjustment of its length. The slow wave structure permits lengths of the meander line to be switched in or out of the circuit quickly and with negligible loss, in order to change the effective length of the antenna. This switching is possible because active switching devices are located between the high and low impedance sections of the meander line. This keeps the current level through the switching device low and results in very low dissipation losses in the switch, thereby maintaining high antenna efficiency.
The MLA allows the physical dimensions of antennas to be significantly reduced while maintaining an electrical length that is still a multiple of a quarter wavelength. Antennas and radiating structures built using this design operate in the region where the limitation on their fundamental performance is governed by the Chu-Harrington relation. Meander line loaded antennas achieve the efficiency limit of the Chu-Harrington relation while allowing the antenna size to be much less than a quarter wavelength at the frequency of operation. Height reductions of 10 to 1 can be achieved over quarter wave monopole antennas while achieving comparable gain.
The prior art MLA antennas have relatively narrow instantaneous bandwidth. Although the switchable meander line allows the antennas to have a very wide tunable bandwidth, the bandwidth available for simultaneous or instantaneous use is relatively limited. Thus for multi-band or multi-use applications and for applications where signals can appear unexpectedly over a wide frequency range, existing MLA antennas are somewhat limited.
Further, as the use of wireless signaling proliferates across the useable spectrum and especially on mobile platforms, the need for wide band or multi-band antennas will only grow in response to the requirement for aperture and volumetric efficiency for the antennas of such systems.
It is, therefore, an object of the invention to provide a meander line loaded antenna (MLA) having a wide instantaneous bandwidth.
It is a still further object of the invention to provide such an antenna which may be replicated in different sizes to create multi-band antennas.
Accordingly, a wide band, meander line loaded antenna includes a first planar conductor extending orthogonally from a ground plane, a signal coupling device connected to the first planar conductor proximally to the ground plane, a second planar conductor substantially parallel to the ground plane and separated from the first planar conductor by a gap, a meander line interconnecting the first and second planar conductors across the gap, and a third conductor connecting the second planar conductor to ground.
Alternatively, the present antenna may be arranged in opposed pairs, and also as two orthogonally opposed pairs for functioning as a quadrature antenna.
Further, each of the antennas, whether singular, opposed pair or quadrature may be replicated as a smaller version and mounted on top of the original with the second planar conductor of the original antenna functioning as the ground plane for the smaller version.